1. Field of the Invention
The present invention relates to optical fiber systems. Especially the present invention relates to monitoring optical states of a data channel in an optical system for high bandwidth transparent wavelength division multiplexing (WDM) networks.
2. General Background
The drive for high-bandwidth transparent WDM optical networks has spurred a need to develop new techniques to monitor channel performance and degradation without requiring optoelectronic-optic (OEO) conversion in the data path. Current optical network performance monitoring relies on SONET line terminating elements (LTEs) to determine the bit error rate (BER) and Q-factor from bits interleaved within the SONET frame or simple loss of signal (LOS) using power monitoring fiber taps. Causes of signal-to-noise (SNR) degradation and distortion are calculated by measuring the characteristics of optical network elements (e.g. fiber dispersion) in advance. However, next generation optical networks will be more dynamic (e.g. dynamic wavelength routing) and signals will traverse different complex paths consisting of fibers, amplifiers, optical add/drop multiplexers, optical crossconnects, etc. At any point within the network, the collective group of wavelength channels will have a different history including path and details of network elements that were transversed. Additionally, degradation and environmental changes will make it very difficult to manage these networks based on statically mapped network element properties.
Optical performance monitoring (OPM) is an approach that ideally allows measurement of channel performance and degradation to be performed without knowledge of the origin or transport history of the data. Direct measurements of, for example, BER are difficult as only a small percentage of optical power can be monitored without degrading the through signal. Yet sufficient power must be available for the monitor to be able to perform as well as the end point receiver in determining the BER. An alternative approach is to monitor various qualities of the data (e.g. chromatic dispersion, polarization mode dispersion, crosstalk, jitter, extinction ratio, channel power, SNR) indicative of channel degradation and compute the BER or performance from these measures. It may also be desirable to characterize certain parameters of a data channel for corrective measures (such as dispersion compensation) or for network management purposes such as downgrading a channels bit rate or reporting degradation to the network management system (NMS) for alarm correlation and fault location.
In the OPM approach, the monitoring technique needs to operate on a portion of the signal that has traversed the optical path with the baseband signal of interest. Optical subcarrier multiplexed signals (OSCM) are a promising candidate to fulfill this requirement as they can be placed close to each optical carrier but still out of band. The signal can be monitored without touching the baseband data yet maintaining a strong correlation with the degradation mechanisms. Moreover OSCM can be used to carry control information in a circuit switched network or label information in packet based architectures.
In comparison with transmitting control on a separate wavelength, the subcarrier per wavelength approach supports distributed network control with a synchronous recovery of wavelength identification, wavelength power, and control data, using a common circuit. It requires only a single laser at each user transmitter and a single photodetector at each monitoring or detection point. The subcarrier portions of the transmitters and receivers can be fabricated using low cost monolithic-microwave integrated circuit (MMIC) technology that has been developed for wireless communications.
Furthermore, crosstalk due to fiber four-wave mixing is low as there is a single subcarrier per wavelength and the relative power of the subcarrier component is much less than the baseband component of the optical signal. Signal cancellation and fading due to dispersion can be overcome using suppressed carrier receivers and single sideband subcarrier modulation techniques. Monitoring of many subcarrier channels using a single photodetector can be achieved with new high power traveling wave photodetector designs.